Network Working Group M. Bellare Internet-Draft T. Kohno Expires: September 20, 2003 UC San Diego C. Namprempre Thammasat University March 20, 2003 SSH Transport Layer Encryption Modes draft-ietf-secsh-newmodes-00.txt Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on September 20, 2003. Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved. Abstract Researchers have recently discovered that the authenticated encryption portion of the current SSH Transport Protocol is vulnerable to several attacks. This document describes new symmetric encryption methods for the SSH Transport Protocol and gives specific recommendations on how Bellare, Kohno, and Namprempre [Page 1] Internet Draft Month, Year frequently SSH implementations should rekey. Bellare, Kohno, and Namprempre [ACM CCS 2002] prove that if an SSH application implements the modifications described in this document, then the symmetric cryptographic portion of that application will provably resist chosen-plaintext, chosen-ciphertext, reaction-based privacy and integrity/authenticity attacks. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Conventions Used in This Document . . . . . . . . . . . . . . 2 3. Rekeying . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3.1 First Rekeying Recommendation . . . . . . . . . . . . . . . . 3 3.2 Second Rekeying Recommendation . . . . . . . . . . . . . . . . 3 4. Encryption Modes . . . . . . . . . . . . . . . . . . . . . . . 4 5. Security Considerations . . . . . . . . . . . . . . . . . . . 6 5.1 Rekeying Considerations . . . . . . . . . . . . . . . . . . . 7 5.2 Encryption Method Considerations . . . . . . . . . . . . . . . 8 References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10 Full Copyright Statement . . . . . . . . . . . . . . . . . . . 10 1. Introduction The symmetric portion of the SSH Transport Protocol was designed to provide both privacy and integrity of encapsulated data. Researchers ([DAI,BKN]) have, however, recently identified several security problems with the symmetric portion of the SSH Transport Protocol as described in [SSH-TRANS]. For example, the encryption mode specified in [SSH-TRANS] is vulnerable to a chosen-plaintext privacy attack. Additionally, if not rekeyed frequently enough, the SSH Transport Protocol may leak information about payload data. This latter property is true regardless of what encryption mode is used. In [BKN] Bellare, Kohno, and Namprempre show how to modify the symmetric portion of the SSH Transport Protocol so that it provably preserves privacy and integrity against chosen-plaintext, chosen- ciphertext, and reaction attacks. This document instantiates the recommendations described in [BKN]. 2. Conventions Used in This Document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. The used data types and terminology are specified in the architecture Bellare, Kohno, and Namprempre [Page 2] Internet Draft Month, Year document [SSH-ARCH]. The SSH Transport Protocol is specified in the transport document [SSH-TRANS]. 3. Rekeying Section 7 of [SSH-TRANS] suggests that SSH implementations rekey after every gigabyte of transmitted data. [SSH-TRANS] does not, however, discuss all the problems that could arise if an SSH implementation does not rekey frequently enough. This section serves to strengthen the suggestion in [SSH-TRANS] by giving firm upper bounds on the tolerable number of encryptions between rekeying operations. In Section 5 we discuss the motivation for these rekeying recommendations in more detail. This section makes two recommendations. Informally, the first recommendation is intended to protects against possible information leakage through the MAC tag and the second recommendation is intended to protect against possible information leakage through the block cipher. Note that, depending on the block length of the underlying block cipher and the length of the encrypted packets, the first recommendation may supersede the second recommendation, or visa- versa. 3.1 First Rekeying Recommendation Because of possible information leakage through the MAC tag, SSH implementations SHOULD rekey at least once every 2**32 outgoing packets. More explicitly, after a key exchange an SSH implementation SHOULD NOT send more than 2**32 packets before rekeying again. SSH implementations SHOULD also attempt to rekey before receiving more than 2**32 packets since the last rekey operation. The preferred way to do this is to rekey after receiving more than 2**31 packets since the last rekey operation. 3.2 Second Rekeying Recommendation Because of a birthday property of block ciphers and some modes of operation, implementations must be careful not to encrypt too many blocks with the same encryption key. Let L be the block length (in bits) of an SSH encryption method's block cipher (eg, 128 for AES). If L is at least 128 then, after rekeying, an SSH implementation SHOULD NOT encrypt more than 2**(L/4) blocks before rekeying again. If L is at least 128, then SSH implementations should also attempt to force a rekey before receiving Bellare, Kohno, and Namprempre [Page 3] Internet Draft Month, Year more than 2**(L/4) blocks. If L is less than 128 (which is the case for older ciphers such as 3DES, Blowfish, CAST-128, and IDEA), then, although it may be too expensive to rekey every 2**(L/4) blocks, it is still advisable for SSH implementations to follow the original recommendation in [SSH-TRANS]: rekey at least once every gigabyte of transmitted data. Note that if L is less than or equal to 128, then the recommendation in this subsection supersedes the recommendation in Section 3.1. If an SSH implementation uses a block cipher with a larger block size (eg, Rijndael with 256-bit blocks), then the recommendations in the above paragraph may supersede the recommendations in this paragraph (depending on the lengths of the packets). 4. Encryption Modes This document describes new encryption methods for use with the SSH Transport Protocol. These encryption methods are in addition to the encryption methods described in Section 4.3 of [SSH-TRANS]. Recall from [SSH-TRANS] that the encryption methods in each direction of an SSH connection MUST run independently of each other and that, when encryption is in effect, the packet length, padding length, payload, and padding fields of each packet MUST be encrypted with the chosen method. Further recall that the total length of the concatenation of the packet length, padding length, payload, and padding MUST be a multiple of the cipher's block size when the cipher's block size is greater than or equal to 8 bytes (which is the case for all of the following methods). This document describes the following new methods: aes128-ctr RECOMMENDED AES (Rijndael) in SDCTR mode, with 128-bit key aes192-ctr RECOMMENDED AES with 192-bit key aes256-ctr RECOMMENDED AES with 256-bit key 3des-ctr RECOMMENDED Three-key 3DES in SDCTR mode blowfish-ctr RECOMMENDED Blowfish in SDCTR mode twofish128-ctr RECOMMENDED Twofish in SDCTR mode, with 128-bit key twofish192-ctr OPTIONAL Twofish with 192-bit key twofish256-ctr OPTIONAL Twofish with 256-bit key serpent128-ctr RECOMMENDED Serpent in SDCTR mode, with with 128-bit key serpent192-ctr OPTIONAL Serpent with 192-bit key serpent256-ctr OPTIONAL Serpent with 256-bit key idea-ctr OPTIONAL IDEA in SDCTR mode cast128-ctr OPTIONAL CAST-128 in SDCTR mode Bellare, Kohno, and Namprempre [Page 4] Internet Draft Month, Year The label -ctr means that the block cipher is to be used in "stateful-decryption counter" (SDCTR) mode. Let L be the block length of in bits. In stateful-decryption counter mode both the sender and the receiver maintain an internal L-bit counter X. The initial value of X should be the initial IV (as computed in Section 5.2 of [SSH-TRANS]) interpreted as an L-bit unsigned integer in network-byte-order. If X=(2**L)-1, then "increment X" has the traditional semantics of "set X to 0." We use the notation to mean "convert X to an L-bit string in network- byte-order." Naturally, implementations may differ in how the internal value X is stored. For example, implementations may store X as multiple unsigned 32-bit counters. To encrypt a packet P=P1||P2||...||Pn (where P1, P2, ..., Pn are each blocks of length L), the encryptor first encrypts with to obtain a block B1. The block B1 is then XORed with P1 to generate the ciphertext block C1. The counter X is then incremented and the process is repeated for each subsequent block in order to generate the entire ciphertext C=C1||C2||...||Cn corresponding to the packet P. Note that the counter X is not included in the ciphertext. Also note that the keystream can be pre-computed and that encryption is parallelizable. To decrypt a ciphertext C=C1||C2||...||Cn, the decryptor (who also maintains its own copy of X), first encrypts its copy of with to generate a block B1 and then XORs B1 to C1 to get P1. The decryptor then increments its copy of the counter X and repeats the above process for each block to obtain the plaintext packet P=P1||P2||...||Pn. As before, the keystream can be pre-computed and decryption is parallelizable. The "aes128-ctr" method uses AES (the Advanced Encryption Standard, formerly Rijndael) with 128-bit keys [AES]. The block size is 16 bytes. The "aes192-ctr" method uses AES with 192-bit keys. The "aes256-ctr" method uses AES with 256-bit keys. The "3des-ctr" method uses three-key triple-DES (encrypt-decrypt- encrypt), where the first 8 bytes of the key are used for the first encryption, the next 8 bytes for the decryption, and the following 8 bytes for the final encryption. This requires 24 bytes of key data (of which 168 bits are actually used). The block size is 8 bytes. This algorithm is defined in [SCHNEIER]. The "blowfish-ctr" method uses Blowfish with 256 bit keys [SCHNEIER]. The block size is 8 bytes. Bellare, Kohno, and Namprempre [Page 5] Internet Draft Month, Year The "twofish128-ctr" method uses Twofish with 128-bit keys [TWOFISH]. The block size is 16 bytes. The "twofish192-ctr" method uses Twofish with 192-bit keys. The "twofish256-ctr" method uses Twofish with 256-bit keys. The "serpent128-ctr" method uses the Serpent block cipher [SERPENT] with 128-bit keys. The block size is 16 bytes. The "serpent192-ctr" method uses Serpent with 192-bit keys. The "serpent256-ctr" method uses Serpent with 256-bit keys. The "idea-ctr" method uses the IDEA cipher [SCHNEIER]. IDEA is patented by Ascom AG. The block size is 8 bytes. The "cast128-ctr" method uses the CAST-128 cipher [RFC2144]. The block size is 8 bytes. 5. Security Considerations This document describes additional encryption methods and recommendations for the SSH Transport Protocol [SSH-TRANS]. [BKN] prove that if an SSH application incorporates the methods and recommendations described in this document, then the symmetric cryptographic portion of that application will resist a large class of privacy and integrity attacks. This section is designed to help implementors understand the security-related motivations for, as well as possible consequences of deviating from, the methods and recommendations described in this document. Additional motivation and discussion, as well as proofs of security, appear in the research paper [BKN]. Please note that the notion of "prove" in the context of [BKN] is that of practice-oriented reductionist security: if an attacker is able to break the symmetric portion of the SSH Transport Protocol using a certain type of attack (eg, a chosen-ciphertext attack), then the attacker will also be able to break one of the transport protocol's underlying components (eg, the underlying block cipher or MAC). If we make the reasonable assumption that the underlying components (such as AES and HMAC-SHA1) are secure, then the attacker against the symmetric portion of the SSH protocol cannot be very successful (since otherwise there would be a contradiction). Please see [BKN] for details. In particular, attacks are not impossible; just extremely improbable (unless the building blocks, like AES, are insecure). Bellare, Kohno, and Namprempre [Page 6] Internet Draft Month, Year Note also that cryptography often plays only a small (but critical) role in an application's overall security. In the case of the SSH Transport Protocol, even though an application might implement the symmetric portion of the SSH protocol exactly as described in this document, the application may still be vulnerable to non-protocol- based attacks (as an egregious example, an application might save cryptographic keys in cleartext to an unprotected file). Consequently, even though the methods described herein come with proofs of security, developers must still execute caution when developing applications that implement these methods. 5.1 Rekeying Considerations Section 3 of this document makes two rekeying recommendations: (1) rekey at least once every 2**32 packets and (2) rekey after a certain number of encrypted blocks (eg, 2**(L/4) blocks if the block cipher's block length L is at least 128 bits). The motivations for recommendations (1) and (2) are different, and we consider each recommendation in turn. Briefly, (1) is designed to protect against information leakage through the SSH protocol's underlying MAC and (2) is designed to protect against information leakage through the SSH protocol's underlying encryption scheme. Please note that, depending on the encryption method's block length L and the number of blocks encrypted per packet, recommendation (1) may supersede recommendation (2) or visa-versa. Recommendation (1) states that SSH implementations should rekey at least once every 2**32 packets. As [BKN] show, if more than 2**32 packets are encrypted and MACed by the SSH Transport Protocol between rekeyings, then the SSH Transport Protocol's underlying MAC may begin to leak information about the protocol's payload data. In more detail, an adversary looks for a collision between the MACs associated to two packets that were MACed with the same 32-bit sequence number (see Section 4.4 of [SSH-TRANS]); if a collision is found, then the payload data associated with those two ciphertexts is probably identical. Note that this problem occurs regardless of how secure the underlying encryption method is. Implementors who decide not to rekey at least once every 2**32 packets should understand this issue. Note that compressing payload data before encrypting and MACing will not significantly reduce the risk of information leakage through the underlying MAC. Similarly, the use of random (and unpredictable to an adversary) padding will not prevent information leakage through the underlying MAC [BKN]. One alternative to recommendation (1) would be to make the SSH Transport Protocol's sequence number more than 32 bits long. This Bellare, Kohno, and Namprempre [Page 7] Internet Draft Month, Year document does not suggest increasing the length of the sequence number because doing so could hinder interoperability with older version of the SSH protocol. Another alternative to recommendation (1) would be to switch from HMAC to a privacy-preserving randomized MAC. Recommendation (2) states that SSH implementations should rekey before encrypting more than 2**(L/4) blocks with the same key (assuming L is at least 128). This recommendation is designed to minimize the risk of birthday attacks against the encryption method's underlying block cipher. For example, there is a theoretical privacy attack against stateful-decryption counter mode if an adversary is allowed to encrypt approximately 2**(L/2) messages with the same key. It is because of these birthday attacks that implementors are highly encouraged to use secure block ciphers with large block lengths. 5.2 Encryption Method Considerations Researchers have recently shown that the original CBC-based encryption methods in [SSH-TRANS] are vulnerable to chosen-plaintext privacy attacks [DAI,BKN]. The new stateful-decryption counter mode encryption methods described in Section 4 of this document were designed to be secure replacements to the original encryption methods described in [SSH-TRANS]. Many people shy away from counter mode-based encryption schemes because, when used incorrectly (such as when the keystream is allowed to repeat), counter mode can be very insecure. Fortunately, the common concerns with counter mode do not apply to SSH because of the rekeying recommendations and because of the additional protection provided by the transport protocol's MAC. This discussion is formalized with proofs of security in [BKN]. As an additional note, when one of the stateful-decryption counter mode encryption methods (Section 4) is used, then the padding included in an SSH packet (Section 4 of [SSH-TRANS]) need not be (but can still be) random. This eliminates the need to generate cryptographically-secure pseudorandom bytes for each packet. One property of counter mode encryption is that it does not require messages to be padded to a multiple of the block cipher's block length. Although not padding messages can reduce the protocol's network consumption, this document requires padding to be a multiple of the block cipher's block length in order to (1) not alter the packet description in [SSH-TRANS] and (2) not leak precise information about the length of the packet's payload data. (Although there may be some networks savings for padding to only 8-bytes even if the block cipher uses 16-byte blocks, because of (1) we do not Bellare, Kohno, and Namprempre [Page 8] Internet Draft Month, Year make that recommendation here.) In addition to stateful-decryption counter mode, [BKN] describe other provably-secure encryption methods for use with the SSH Transport Protocol. The stateful-decryption counter mode methods in Section 4 are, however, the preferred alternatives to the insecure methods in [SSH-TRANS] because stateful-decryption counter mode is the most efficient (both in terms of network consumption and in terms of the number of required cryptographic operations per packet). References [AES] Daemon, J. and Rijmen, V., "AES Proposal: Rijndael", NIST AES Proposal, 1998. [BKN] Bellare, M., Kohno, T., and Namprempre, C., "Authenticated Encryption in SSH: Provably Fixing the SSH Binary Packet Protocol", Ninth ACM Conference on Computer and Communications Security, 2002. [BN] Bellare, M. and Namprempre, C., "Authenticated Encryption: Relations among notions and analysis of the generic composition paradigm", Asiacrypt 2000. [DAI] Dai, W., "An Attack Against SSH2 Protocol", Email to the ietf-ssh@netbsd.org email list, 2002. [KRAWCZYK] Krawczyk, H., "The Order of Encryption and Authentication for Protecting Communications (Or: How secure is SSL?)", Crypto 2001. [RFC2119] Bradner, S., "Key Words for Use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2144] Adams, C., "The CAST-128 Encryption Algorithm", RFC 2144, May 1997. [SCHNEIER] Schneier, B., "Applied Cryptography Second Edition: Protocols algorithms and source in code in C", Wiley, 1996. [SERPENT] Anderson, R., Biham, E., and Knudsen, L. "Serpent: A proposal for the Advanced Encryption Standard", NIST AES Proposal, 1998. [SSH-ARCH] Ylonen, T., et. al., "SSH Protocol Architecture", I-D draft-ietf-architecture-12.txt, January 2002. Bellare, Kohno, and Namprempre [Page 9] Internet Draft Month, Year [SSH-TRANS] Ylonen, T., et. al., "SSH Transport Layer Protocol", I-D draft-ietf-transport-14.txt, March 2002. [TWOFISH] Schneier, B., et. al., "The Twofish Encryptions Algorithm: A 128-bit block cipher, 1st Edition", Wiley, 1999. Authors' Addresses: Mihir Bellare Department of Computer Science and Engineering University of California at San Diego 9500 Gilman Drive, MC 0114 La Jolla, CA 92093-0114 Phone: +1 858-822-2977 EMail: mihir@cs.ucsd.edu Tadayoshi Kohno Department of Computer Science and Engineering University of California at San Diego 9500 Gilman Drive, MC 0114 La Jolla, CA 92093-0114 Phone: +1 858-822-2977 EMail: tkohno@cs.ucsd.edu Chanathip Namprempre Thammasat University Faculty of Engineering Electrical Engineering Department Rangsit Campus, Klong Luang Pathumthani, Thailand 12121 EMail: meaw@alum.mit.edu Full Copyright Statement Copyright (C) The Internet Society (2003). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Bellare, Kohno, and Namprempre [Page 10] Internet Draft Month, Year Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Acknowledgments Mihir Bellare and Chanathip Namprempre were supported by NSF Grant CCR-0098123, NSF Grant ANR-0129617 and an IBM Faculty Partnership Development Award. Tadayoshi Kohno was supported by a National Defense Science and Engineering Fellowship. Bellare, Kohno, and Namprempre [Page 11]